1. Field of the Invention
The present invention relates to a dot position measurement method, a dot position measurement apparatus, and a computer readable medium, and more particularly to dot position measurement technique suitable for measurement of a deposition position of a dot recorded by each nozzle of an inkjet head.
2. Description of the Related Art
One method of recording an image onto a recording medium such as recording paper is an inkjet drawing method in which an image is recorded by ejecting ink droplets in response to an image signal and causing the ink droplets to impact on the recording medium. As an image forming apparatus which employs such an inkjet drawing system, there exists a full-line head image drawing apparatus, in which an ejection unit (nozzle) which ejects ink droplets, is disposed in a line facing the whole of one side of the recording medium, and the recording medium is conveyed in a direction orthogonal to the ejection unit so as to record an image over the whole area of recording medium.
By conveying the recording medium without moving the ejection unit, the full-line head image drawing apparatus is able to draw an image over the whole area of the recording medium and increase the recording speed.
However, with line-head image forming apparatuses, there is the problem that streaks or unevenness of the image recorded on the recording medium occurs due to inconsistencies during production such as displacement of the ejection unit.
Such streaks and unevenness are caused by scatter of the ink droplet impact position, and techniques to correct streaks and unevenness, based on the impact position, are known.
Japanese Patent Application Publication No. 2008-44273 discloses a technology whereby a line pattern and, at the same time, a reference pattern are read with a scanner, and the impact position is measured while correcting any scanner conveyance errors.
Japanese Patent Application Publication No. 2008-80630 discloses a technology which reads a line pattern with a scanner to determine the edge position of a line from the read image, and measure the line position (impact position) from a plurality of edge positions for each line.
In recent years, as paper widths have grown larger and higher line-head densities have been developed, the number of nozzles to be measured has reached the tens of thousands or more. For example, a recording width of eleven inches at a resolution of 1200 DPI requires 13200 nozzles per ink, and for the four inks of the CMYK color model, there are a total of 52800 nozzles. A print head with such a large number of nozzles requires a high-speed, high-accuracy, and low-cost impact position measurement method.
More specifically, taking a 1200-DPI image drawing apparatus as an example, the recording lattice pitch for 1200 DPI is 21.17 μm, and a dot diameter equal to or more than 21.17×√2 is required to deposit dots gaplessly, and therefore a dot diameter of approximately 30 to 40 μm is required.
4800 DPI is about the upper limit for commercial scanners, even for high-resolution scanners, and, at this resolution, the reading lattice pitch of the scanner is approximately 5.29 μm. In comparison with the dot diameter, the impact position must be found from as many as 6 to 8 pixels. These figures are cut in half for 2400 DPI. Although higher resolutions are desirable for reading devices (scanners) in order to improve impact position accuracy, higher reading device resolutions cause (1) problems with the size of read image data, and (2) the problem that reading is not completed in a single pass.
Suppose, for example, that, for a reading resolution of 4800 DPI, the size of the impact position precision measurement sample is A3-size, the A3 reading range is then 11.5 inches×15.5 inches, which means that, for a color image, the total data amount of the read image, for the 8 bits on each of the three RGB channels, is 12.3 GB. The reading resolution is 3.08 GB even for 2400 DPI. Such a large volume of data is time-consuming even when the data is read to a hard disk device (HDD).
Moreover, since current commercial scanners have a limited reading range at the highest resolution (4800 DPI for an A4 scanner and 2400 DPI for an A3 scanner, for example), reading cannot be performed all at once at the maximum reading range. The maximum reading range must therefore be divided into strips for reading.
Thus, in cases where a single image is divided up for reading, each initial scanner operation (the time taken to correct the brightness, and the time to move to the designated reading position) takes time. Typically, an overlap region must be added to the reading range in order to ensure mutual conformity between the data corresponding to the reading regions thus divided. Extra capacity for the image data of the overlap region is required and the reading time is increased to the extent of the overlap region. Typically, the larger the number of divisions of the whole reading range, the greater the proportion of the overlap region to the reading range. Even if processing is performed to reduce the image data and the write time is reduced, dividing up an image causes problems, namely a larger image data capacity, and an increase in the reading time.
The technologies disclosed in Japanese Patent Application Publication Nos. 2008-44273 and 2008-80630 are faced by the problem that, because the main and sub-scanning resolutions during reading are the same, when these technologies are used, an image cannot be read all at once, or the processing time is long due to the large size of the image to be processed.